The present invention relates to a novel method for the production of tetramethylthiuram disulfide, in which dimethylammonium dimethyldithiocarbamate is oxidized in aqueous solution with oxygen, in the presence of carbon disulfide and a lignosulfonate.
Many methods for the production of tetramethylthiuram disulfide (the abbreviated term of TMTD will be used later in the whole description) have been described in the literature. In many of these methods, a dimethyldithiocarbamate salt is first of all prepared in aqueous solution by a reaction between dimethylamine and carbon disulfide in basic medium. The dimethyldithiocarbamate salt thus obtained is then oxidized to TMTD with various oxidants, for example hydrogen peroxide (patent DE 2,527,898), chlorine (U.S. Pat. No. 2,751,415 and U.S. Pat. No. 2,751,416), sodium nitrite (patent DE 1,164,394), and the like. A major disadvantage of most of these methods is that the formation of TMTD is accompanied by the formation of a salt from which it will have to be separated. A use must be found for the salt itself or it will have to be discharged.
Oxygen has more rarely been used as oxidant. Thus, patents FR 1,322,579 and FR 1,322,580 describe 2 very similar methods for the production of substituted thiuram disulfide, the first starting with alkali metal salts of substituted dithiocarbamic acids, the second starting with disubstituted secondary amines and carbon disulfide. In the 2 methods, the oxidation is produced by atmospheric oxygen in the presence of group VIII metal phthalocyanines as catalysts and the solvent may be water, the pH being maintained between 7 and 12. The yield of the first method reaches about 80% maximum, whereas it is limited to about 30% in the second.
In patent BE 892,143, there is described the synthesis of substituted thiuram disulfides by a method in which a disubstituted secondary amine whose pKa is at least 8 is reacted with carbon sulfide in the presence of oxygen or air as oxidant and a metal catalyst. The metals are chosen from cerium, manganese, copper, molybdenum, vanadium or one of their derivatives, and they are used in an amount of 0.01 to 5 millimoles per mole of secondary amine. The method is performed either in a single stage, or in 2 stages, the oxygen being used only in the second stage, and the intermediate dithiocarbamate being isolated or otherwise. The solvent preferably used as reaction medium is an aromatic hydrocarbon, a low-molecular weight alcohol or a mixture of the latter with water. In these organic solvents, the yield is practically quantitative and the products obtained generally possess sufficient purity for it not to be necessary to further purify them. By contrast, in pure water, the reaction is possible but it is much slower and the yield and selectivity are poorer, which constitutes a considerable disadvantage. According to this patent, it is possible to recycle the solvent with its catalyst more then 10 times, with no loss of yield or of catalytic activity. However, it is clear that at a certain moment, it will be necessary to separate the deactivated catalyst from the solvent and to purify the latter, which constitutes a disadvantage.
In patent BE 892,144, there is described a method which is essentially only distinguishable from that of patent BE 892,143 by the fact that ammonia or a tertiary amine is added to the reagents, and that the choice of catalytic metals is wider.
It would be of interest to have a method for the synthesis of TMTD in which:
the yields and selectivities would be practically quantitative;
the reaction medium would be water;
the oxidation would be performed with oxygen, or with a gas containing oxygen;
the formation of TMTD would not be accompanied by the formation of other products, such as salts.
In addition, for agrochemical applications:
the product obtained would be at a sufficiently high concentration in the reaction medium so that the latter can be directly spray-dried, without the need for filtration and washes giving mother liquors whose disposal or treatment would pose problems;
the products present in the reaction mixture would need to be compatible with an agrochemical use, without requiring prior separation (separation of the metal catalysts of patent BE 892,143 or of patents FR 1,322,579 and 1,322,580 for example).
It has been found, surprisingly, that all these combined objectives could be achieved by carrying out the oxidation of dimethylammonium dimethyldithio-carbamate (or DMDTC, a term used throughout this description) with oxygen, the pH being maintained between 7.0 and 8.0, in water and in the presence of a dispersing agent chosen from lignosulfonates.
Accordingly, the present invention relates to a method for the production of tetramethylthiuram disulfide (TMTD), in which dimethylammonium dimethyldithio carbamate (DMDTC) is oxidized in aqueous solution with oxygen, with addition of carbon disulfide, characterized in that the reaction is carried out in the presence of 0.5 to 25% of a lignosulfonate, calculated over the theoretical quantity of tetramethylthiuram disulfide (TMTD) which may be formed, and at a value of the pH of the reaction mixture of between 7.0 and 8.0.
Lignosulfonates are by-products derived from the treatment of paper pulp with sulfites (Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A15, pp. 305-315, VCH Ed., 5 ie. (1990)). Lignosulfonates are polymers based on phenyl propane units. As natural products, their composition varies according to the origin. They contain metals in trace form, in quantities well below the catalytic quantities used, for example, in patents BE 892,143, FR 1,322,579 or FR 1,322,580. Ligndsulfonates are very good dispersants, and that is why they are widely used in agrochemical formulations in the form of wettable powders or water-dispersible granules, to enhance the wettability and the dispersing qualities of active agents present in these formulations. They are marketed in the form of ammonium, sodium, calcium or magnesium salts and the like. By way of examples, there may be mentioned the products marketed under the trade names Wannin AM, Collex XM, Zewa SL, Borresperse NH, Zewakol MGN (all products from the company Borregaard-Lignotec), Novizel C12 (company Avebene), and the like. All these lignosulfonates are suitable for the aims of the present invention.
Normally, the lignosulfonates are used after the synthesis of the active agents, such as TMTD, during the preparation of the final formulations, with other additives. Unexpectedly, the surprising discovery has been made that the addition of these lignosulfonates during the stage of synthesis of active agents such as TMTD, through oxidation under an oxygen atmosphere of aqueous solutions of DMDTC, leads spontaneously to the formation of TMTD.
The DMDTC in aqueous solution, used as a reagent in the method for the production of TMTD according to the present invention, may be obtained by methods well known to persons skilled in the art, for example by reaction between dimethylamine in aqueous solution at 40% and carbon disulfide. Aqueous solutions containing about 55% by weight of DMDTC are easily obtained which are quite suitable as reagents according to the invention.
According to the invention, the lignosulfonate is used in quantities ranging from 0.5 to 25% by weight, relative to the weight of TMTD formed. The upper limit of this quantity is especially of interest in the case where the TMTD produced is intended for formulations of water dispersible granules for agrochemical uses. This quantity of lignosulfonate is such that it is no longer necessary to add more thereof during the formulation. However, it is not necessary to use such high quantities of lignosulfonates in order to obtain all the favorable effects of the present invention, and quantities of 0.5 to 3% by weight, relative to the weight of TMTD formed, are sufficient for this purpose.
The best results are obtained according to the invention if the oxidation of DMDTC is carried out at temperatures of between room temperature and 120xc2x0 C., preferably between 50 and 90xc2x0 C., and at pressure values of between 1 and 20 atmospheres, preferably 2 and 10 atmospheres. The pressure is maintained constant by addition of oxygen up to the end of the oxidation.
A good monitoring of the pH during the oxidation of DMDTC in aqueous solution is an important element in order to obtain a very high selectivity in relation to TMTD. Indeed, the oxidation of DMDTC releases dimethylamine according to the reaction scheme:
2(CH3)2Nxe2x80x94CS2xe2x88x92 +H2N(CH3)2+xc2xd O2xe2x86x92[(CH3)2Nxe2x80x94CSxe2x80x94S]2+H2O +2(CH3)2NH
Through gradual addition of carbon disulfide, the imethylamine released is converted to DMDTC, according to the reaction scheme:
2(CH3)2NH+CS2xe2x86x922(CH3)2Nxe2x80x94CS2xe2x88x92 +H2N(CH3)2
This DMDTC is in turn oxidized to TMTD according to the first reaction.
If a transient excess of carbon disulfide is admitted during the oxidation, there are risks of inhibition of the oxidation, of agglomeration of the TMTD on the reactor surfaces and of explosion of the gaseous phase. If, on the other hand, an excessively low amount of carbon disulfide is admitted, there will be a transient excess of dimethylamine which will result in the formation of by-products and a loss of selectivity in relation to TMTD.
It is therefore important to admit into the oxidation reaction only the exact quantity of carbon disulfide necessary for its reaction with dimethylamine, as the latter is formed. The solution to this problem consists in automatically regulating the flow rate of carbon disulfide as a function of the pH continuously measured in the reaction mixture. Thus, by maintaining the pH at a value of between 7.0 and 8.0, a very good selectivity is obtained. Preferably, the pH is maintained between 7.0 and 7.3, which makes it possible to obtain a selectivity of close to 99.5% in relation to TMTD.
When the reaction mixture no longer absorbs oxygen, the reaction is complete. The last traces of carbon disulfide are removed by entrainment with nitrogen. If the TMTD formed is intended for agrochemical uses, the reaction mixture may be directly spray-dried, as it is, in order to provide TMTD which can be used as a fungicide.
Accordingly, the present invention also relates to a method for the production of tetramethylthiuram disulfide (TMTD) formulations, characterized in that:
TMTD is produced according to the above mentioned method for the production of TMTD;
the reaction mixture obtained according to the above mentioned method is directly spray-dried.
As a variant for agrochemical applications in the form of water dispersible granules, the reaction mixture containing the TMTD suspension may be ground to a sufficiently fine particle size, the customary ingredients such ask additional dispersing agents, wetting agents and inert fillers may be added thereto and the mixture may be granulated by spraying.
Accordingly, the present invention also relates to a method for the production of water dispersible granules containing tetramethylthiuram disulfide (TMTD), characterized in that:
TMTD is produced according to the above mentioned method for the production of TMTD;
the reaction mixture obtained according to the above mentioned method is ground;
the customary ingredients for water dispersible granules are added thereto;
the above mentioned mixture is granulated by spraying.
For other applications, for example in the vulcanization of rubber, the reaction mixture is treated in order to separate the TMTD from the lignosulfonate.
The method for the production of TMTD by oxidation, with oxygen, of DMDTC in aqueous solution, in the presence of lignosulfonates, according to the present invention, exhibits numerous advantages:
the degree of conversion is practically quantitative, with TMTD yields greater than 98%, for a selectivity which may be up to 99.5%;
the practically nonexistent formulation of by-products and the absence of added metal catalyst (in contrast to patents BE 892,143, FR 1,322,579 or FR 1,322,580) makes it possible to use the reaction mixture containing TMTD directly, without prior filtration, to make formulations thereof which can be used in agrochemistry. Filtration cakes and washing liquids are thus avoided, which makes the method economic and not harmful for the environment;
the presence of lignosulfonates in the reaction mixture confers a dispersing and liquefying action on TMTD, which makes it possible to obtain a concentration as high as 40%, and even 45% at the end of the reaction. Without lignosulfonates, it is difficult to exceed a TMTD concentration of 30% in the reaction mixture, because adequate stirring of the TMTD suspension is no longer possible at this concentration. Such a high TMTD concentration facilitates the direct use of the reaction mixtures containing it in agrochemical formulations.
The method according to the invention may be carried out continuously, or in batches. In the latter case, it is possible, for example, to use a reactor withstanding pressure values of 20 atmospheres or more, comprising a stirrer of a type providing good contact between the liquid phase and the gaseous atmosphere above the liquid.
The reactor is first of all charged with an aqueous solution of DMDTC and there are then added the lignosulfonate and water in a quantity so as to obtain, at the end of the reaction, a suspension containing between 35 and 45% TMTD.
Oxygen is then added to a pressure of 1 to 20 atmospheres.
The contents of the reactor are then heated to a sufficiently high temperature to initiate the oxidation reaction.
The temperature is maintained at a fixed value (between room temperature and 120xc2x0 C.) by cooling the reactor.
The initial pressure is maintained by constant addition of oxygen, regulated with a pressure regulator.
The pH of the reaction mixture is maintained between 7.0 and 8.0 by continuous addition of carbon disulfide, regulated by a control valve controlled by a pH-stat signal.
The reaction is stopped when the consumption of oxygen and of carbon disulfide stops.
The reactor is then emptied and the last traces of carbon disulfide are removed by entrainment by means of a nitrogen stream.
The aqueous suspension of TMTD is then spray-dried in a drier, such as for example a spray-drier, a fluidized bed or a xe2x80x9cflash-drierxe2x80x9d.
The following examples illustrate the invention without however limiting it.